A comment from Stefan K during the code review of JDK-8167108:

6) I think it would be nice if the SMR stuff in thread.hpp were encapsulated into an class instead of added directly to Thread and Threads. I sort-of expected the SMR variables to be moved to threadSMR.hpp.

For example:

 class Threads: AllStatic {
   friend class VMStructs;
  private:
+  // Safe Memory Reclamation (SMR) support:
+  static Monitor*              _smr_delete_lock;
+  // The '_cnt', '_max' and '_times" fields are enabled via
+  // -XX:+EnableThreadSMRStatistics:
+  static uint                  _smr_delete_lock_wait_cnt;
+  static uint                  _smr_delete_lock_wait_max;
+  static volatile int          _smr_delete_notify;
+  static volatile jint         _smr_deleted_thread_cnt;
+  static volatile jint         _smr_deleted_thread_time_max;
+  static volatile jint         _smr_deleted_thread_times;
+  static ThreadsList* volatile _smr_java_thread_list;
+  static ThreadsList*          get_smr_java_thread_list() {
+    return (ThreadsList*)OrderAccess::load_ptr_acquire((void* volatile*)&_smr_java_thread_list);
+  }
+  static ThreadsList*          xchg_smr_java_thread_list(ThreadsList* new_list) {
+    return (ThreadsList*)Atomic::xchg_ptr((void*)new_list, (volatile void*)&_smr_java_thread_list);
+  }
+  static long                  _smr_java_thread_list_alloc_cnt;
+  static long                  _smr_java_thread_list_free_cnt;
+  static uint                  _smr_java_thread_list_max;
+  static uint                  _smr_nested_thread_list_max;
+  static volatile jint         _smr_tlh_cnt;
+  static volatile jint         _smr_tlh_time_max;
+  static volatile jint         _smr_tlh_times;
+  static ThreadsList*          _smr_to_delete_list;
+  static uint                  _smr_to_delete_list_cnt;
+  static uint                  _smr_to_delete_list_max;

Could be:

class Threads: AllStatic {
  friend class VMStructs;
 private:
  // Safe Memory Reclamation (SMR) support:
  SMRSupport _smr_support;

And SMRSupport could be moved to threadSMR.hpp.